1. Field of the Invention
The invention relates to a three-phase brushless direct current motor. More particularly, the invention relates to a commutation circuit for a sensorless three-phase brushless direct current motor.
2. Description of Related Technology
In controlling currents through the windings of a three-phase brushless direct current (BLDC) motor, commutation is conventionally accomplished using Hall-effect sensors. The Hall-effect sensors are used to detect the position of the permanent magnets in the motor rotor to provide signals associated with the absolute position of the rotor to the commutation circuitry. Conventional commutation circuits use the phase and amplitude of the Hall-effect sensor signals to soft-switch the current in the motor windings during constant velocity operation of the motor. Additionally, the Hall-effect sensor signals may be used to provide the desired commutation during initial starting of the motor.
FIG. 1 illustrates a typical input stage of an output driving circuit in a conventional three-phase BLDC motor commutation circuit. The input stage includes three differential amplifiers 10, 20, 30 that receive signal pairs U_Hall+/U_Hall-, V_Hall+/V_Hall-, W_Hall+/W_Hall- from three respective Hall-effect sensors (not shown). The phase difference between the signals of each signal pair is 180.degree..
Where the difference between the individual signals of the signal pairs is less than 100 mV (i.e., approximately four times the thermal voltage V.sub.T), the output currents I.sub.c 1-I.sub.c 6 of the differential amplifiers 10, 20, 30 are linearly increased or decreased. Where the difference between the individual signals of the signal pairs is greater than 100 mV, the output currents Ic1-Ic6 of the differential amplifiers 10, 20, 30 are limited by the current source I.sub.EE. Because the output currents I.sub.c 1-I.sub.c 6 of the differential amplifiers 10, 20, 30 are linearly increased or decreased for signal pair differences in an interval surrounding zero volts, the output stage of the output driving circuit can be soft switched in the interval surrounding zero volts. Soft switching prevents the excitation current of the stator coil of the motor from changing rapidly, thereby allowing the motor to rotate freely without sparks.
In contrast to the above-described conventional commutation circuit, which uses Hall-effect sensors, a sensorless three-phase BLDC motor commutation circuit uses back electromotive force EMF, which is generated in the unexcited phase of a stator coil, instead of Hall-effect signals. In particular, a zero crossing point at which the back EMF and neutral point voltage intersect is detected, and commutation is accomplished by using the zero crossing point to determine the absolute position of the motor rotor.
FIG. 2 illustrates a timing diagram that shows the time domain relationship between the conventional commutation signals of a three-phase BLDC motor having three Hall-effect sensors and the zero crossing point of a sensorless three-phase BLDC motor. Detail (a) in FIG. 2 highlights a region in which the voltage difference between the signals of the U_Hall signal pair is less than 100 mV, thereby allowing conventional commutation to accomplish soft switching within the highlighted region. Detail (b) highlights a region surrounding the point at which the back EMF U_BEMF and the neutral point N intersect for a sensorless three-phase BLDC motor. As shown by detail (b) of FIG. 2, when the motor is driving with a constant velocity, the zero crossing point electrically leads the conventional commutation soft switching point by 30.degree.. Thus, to optimize commutation in a sensorless three-phase BLDC motor using the back EMF signal, the phase of the back EMF signal must be electrically delayed by 30.degree. before it is used in conjunction with the zero crossing point to perform commutation.
Generally, a complex delay circuit having a plurality of delay elements is needed to delay the back EMF signal. Furthermore, the combination of the current flowing through the motor windings and the resistance of the windings introduces substantial non-ideal voltages that make detection of the back EMF signal difficult and prone to error. Still further, a commutation mode using back EMF signals is typically a hard switching mode, which may produce undesirable sparks together with electromagnetic interference (EMI) in the switching process. While a snubber circuit may used to suppress the sparking and the EMI, this further increases the complexity of the delay circuit.
Additional complications arise in starting a stopped sensorless three-phase BLDC motor because the back EMF signal exhibits a poor signal-to-noise ratio and is an unreliable indicator of the rotor position until the motor has a reached a sufficient rotor speed. As a result, if commutation is started by using information from the back EMF signal that is detected during the forcible start of the motor an unstable commutation could result, which could stop rotation of the motor or rotate the rotor in an undesirable direction.